The invention relates to a process for the low-temperature (cryogenic) fractionation of air in a distillation column system for nitrogen-oxygen separation, in which
a first feed air stream is introduced into the distillation column system for nitrogen-oxygen, separation,
an oxygen-rich fraction from the distillation column system for nitrogen-oxygen separation is pressurized in liquid form and is added to a mixing column,
a heat transfer medium stream, in particular a second feed air stream, is introduced into the lower region of the mixing column and is brought into countercurrent contact with the oxygen-rich fraction,
a gaseous top product is obtained in the upper region of the mixing column, and
a liquid from the lower or middle region of the mixing column is introduced into the distillation column system for nitrogen-oxygen separation.
The distillation column system for nitrogen-oxygen separation of the invention may be designed as a two-column system, for example as a conventional double column system, but may also be designed as a one-column, three-column or multi-column system. In addition to the columns for nitrogen-oxygen separation, it may include further equipment for obtaining other constituents of air, in particular noble gases (for example argon).
The oxygen-rich fraction which is used as feed for the mixing column has an oxygen concentration which is higher than that of air and is, for example, 70 to 99.5 mol %, preferably 90 to 98 mol %. A mixing column is understood as meaning a countercurrent contact column in which a relatively highly volatile gaseous fraction is passed in the opposite direction to a liquid of lower volatility.
The process according to the invention is particularly suitable for obtaining gaseous pressurized impure oxygen. In this context, the term impure oxygen is understood as meaning a mixture with an oxygen content of 99.5 mol % or less, in particular of 70 to 99.5 mol %. The product pressures are, for example, 2.2 to 4.9 bar, preferably 2.5 to 4.5 bar. Of course, if necessary the pressurized product can be compressed further in the gaseous state. In principle, the invention can also be employed at mixing column pressures which are higher than the high-pressure column pressure, for example 4.5 to 16 bar, in particular 5 to 12 bar.
Processes of the type described in the introduction are known from EP 531182 A1, DE 19951521 A1 and EP 1139046 A1. Although it is mentioned in passing in EP 1139046 A1 that a krypton-xenon recovery means may be connected downstream of a mixing column system of this type, in practice this has not hitherto been implemented, since with a system of this nature the methods which have hitherto been customary have been unable to achieve economically viable yields of krypton and xenon.
An object of the invention is therefore to provide a process of the type described in the introduction and a corresponding apparatus which operate particularly economically and in particular have a relatively high krypton and/or xenon yield.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
These objects are achieved by introducing a krypton- and xenon-containing oxygen stream from the distillation column system for nitrogen-oxygen separation into a krypton-xenon enriching column, a krypton- and xenon-enriched fraction is obtained in the krypton-xenon enriching column, and that the gaseous top product of the mixing column is introduced into an additional column, in the upper region of which a krypton- and xenon-depleted top fraction is obtained.
The krypton-xenon enriching column fulfils the standard function of a krypton and xenon enriching means while, at the same time, rejecting methane. However, this alone is not sufficient to achieve a satisfactory krypton and xenon yield in a mixing column process. This is because a significant proportion of the relatively low-volatility constituents of the air is normally removed from the process together with the top product of the mixing column.
Therefore, in the invention in addition to the krypton-xenon enriching column there is an additional column which retains the krypton and xenon which are still present in the top product of the mixing column. This valuable product is therefore no longer lost with the pressurized oxygen product, but rather, by way of example, can be returned to the mixing column or the distillation column system and, from there, can be introduced into the krypton-xenon enriching column.
The terms xe2x80x9cmixing columnxe2x80x9d and xe2x80x9cadditional columnxe2x80x9d are in the present context in each case understood in functional terms to represent corresponding countercurrent mass transfer zones. They may be, but do not have to be, arranged in separate vessels. In particular, it is possible for two or more zones of this type to be located one above the other in a common vessel if they are at similar pressure levels. In the invention, by way of example, the mixing column and the additional column may be produced as a combined column of this type. Alternatively, a horizontal partition may be installed between mixing column and additional column, or mixing column and additional column may be accommodated in completely separate vessels.
It is preferable for a part of the krypton- and xenon-depleted top fraction from the additional column to be obtained as gaseous pressurized oxygen product without krypton and xenon being lost in significant amounts.
Furthermore, it is favourable if a (futher) part of the krypton- and xenon-depleted top fraction from the additional column is condensed in a condenser-evaporator. The condensate which is generated in the condenser-evaporator is substantially free of krypton and xenon and is used as reflux for the additional column and the krypton-xenon enriching column.
The condenser-evaporator can simultaneously serve as a bottom evaporator of the krypton-xenon enriching column. The additional column and krypton-xenon enriching column therefore form the high-pressure and low-pressure parts, respectively, of a double column.
In a preferred configuration of the process according to the invention, the oxygen-rich fraction which is added to the mixing column is removed one to five theoretical plates, preferably two to four theoretical plates, above the bottom of the or one of the columns of the distillation column system for nitrogen-oxygen separation. This fraction generally originates from a corresponding intermediate location of the low-pressure column of a two-column system. The krypton- and xenon-containing oxygen stream for the krypton-xenon enriching column, by contrast, is withdrawn from the bottom of this column.
The invention also relates to an apparatus for the low-temperature fractionation of air comprising a distillation column system for nitrogen-oxygen separation, having a mixing column and
having a first feed air line, which is connected to the distillation column system for nitrogen-oxygen separation,
having a first liquid oxygen line, which is connected to the distillation column system for nitrogen-oxygen separation and leads into the mixing column via means for increasing the pressure of the liquid,
having a heat transfer medium line, in particular a second feed air line, which leads into the lower region of the mixing column,
having means for obtaining a gaseous top product in the upper region of the mixing column, and
having a liquid line which leads out of the lower or middle region of the mixing column into the distillation column system for nitrogen-oxygen separation,
a krypton-xenon enriching column for obtaining a krypton- and xenon-enriched fraction,
a second liquid oxygen line for introducing a krypton- and xenon-containing oxygen stream from the distillation column system for nitrogen-oxygen separation into the krypton-xenon enriching column,
means for introducing the gaseous top product of the mixing column into an additional column, and
means for obtaining a krypton- and xenon-depleted top fraction in the upper region of the krypton-xenon enriching column.